The present invention relates to a vulcanizable rubber composition of the type comprising a cross-linkable unsaturated chain polymeric base, which has a preferred, while not exclusive, use in the manufacture of treads for vehicle tires.
The invention also relates to a tread and a tire obtainable by the above composition.
In the following description and in the subsequent claims, the terms: cross-linkable unsaturated chain polymeric base, are used to indicate any non cross-linked polymer, either natural or synthetic, capable of assuming all the physico-chemical and mechanical characteristics typical of elastomers as a result of cross-linking (vulcanization).
As is known, in the manufacture of vehicle tires and, more particularly, of tires having very high performances, one of the more difficult problems to solve has always been that of achieving either a low rolling resistance at the temperatures of normal use of the tire (40-70xc2x0 C.), or an adequate road holding (grip) when the tire exceeds said temperatures as a result of high thermo-mechanical stresses, such as, for instance, those ensuing from a so-called xe2x80x9cborderlinexe2x80x9d use of said tire.
It is also known that the difficulty of simultaneously achieving these desired characteristics essentially stems from the fact that they are affected in a completely opposite manner by the hysteretic characteristics, i.e. by the amount of dissipated energy, of the rubber composition that constitutes the tire tread.
In order to achieve the desired low rolling resistance of the tire, in fact, it is necessary to use rubber compositions with a low hysteresis value, and as such suitable to dissipate limited energy amounts during rolling.
On the other hand, in order to achieve the desired high road grip, it is necessary to use rubber compositions having a high hysteresis value, and as such suitable to dissipate energy amounts apt to ensure a high grip between the tread and the ground.
The solution to this problem is even more difficult if it is considered that the rubber composition constituting the tire tread should possess hysteretic behaviours quite opposite and incompatible with one another, depending upon the working temperature range of the tire.
All the attempts made in the art to improve tire performances within a temperature range above those normally used, in fact, have met either a marked and undesired increase in rolling resistance, or a substantial worsening of the abrasion resistance characteristics of the tread.
These disappointing attempts were substantially based on the use in the rubber composition of hydrocarbon resins substantially soluble in the SBR-containing polymeric bases tipically used in the production of treads for vehicle tires, such as for example alpha-methylstyrene-derived resins (such as the resin marketed under the trade name KRISTALEX(trademark) F-85, produced by Hercules, Wilmington, Del. U.S.A.) or coumarone-based resins.
The ensuing practical consequence is that the above problem has not been solved yet in the art.
As a consequence, the technical problem underlying the present invention is that of providing a rubber composition having a different hysteretic behaviour depending upon the working temperature of the tire, so as to obtain either a low rolling resistance at the temperatures of normal use of the tire (40-70xc2x0 C.), or an adequate road holding (grip) when the tire exceeds said temperatures as a result of high thermo-mechanical stresses.
According to the present invention, it has surprisingly been found that the desired low rolling resistances at the above working temperatures of the tire and high grip in borderline working conditions of the same, may be simultaneously achieved provided that at least one organic compound, solid at room temperature, is incorporated in the polymeric base of the rubber composition used to manufacture the tread, which compound:
a) should be substantially insoluble in said polymeric base; and
b) should have a first or second order transition temperature of from 80 to 160xc2x0 C.
Preferably, said organic compound is selected from the group comprising: amorphous or semicrystalline polymers, oligomers, low-molecular weight organic substances and mixtures thereof.
Preferably, the polymers of the invention have an average numeric molecular weight of from 10,000 to 1,000,000.
For the purposes of the invention, polymers of preferred and advantageous use are those selected from the group comprising: polyphenylenether, polyethylene, polypropylene, polymethyl-methacrylate, polystyrene, polyvinylalcohol, ethylene/vinyl alcohol copolymers, acrylonitrile-butadiene-styrene (ABS) terpolymers, ethylene-methacrylic acid copolymers, styrene-isoprene (SIS), styrene-butadiene (SBS), styrene-ethylene/butylene-styrene (S-E/B-S), and styrene-ethylene/propylene (S-E/P) block copolymers and mixtures thereof.
Optimum results have been achieved with polyphenylenether having a glass transition temperature of 120xc2x0 C., marketed by Hxc3xcls under the trade name VESTORAN(trademark) 1100.
Preferably, the oligomers of the invention have an average numeric molecular weight of less than 10,000, and more preferably of from 500 to 10,000.
For the purposes of the invention, oligomers of preferred and advantageous use are those selected from the group comprising aliphatic or aromatic hydrocarbon resins having an average numeric molecular weight comprised in the above-defined range.
Optimum results have been achieved either with the aliphatic hydrocarbon resins marketed by Hercules under the trade name PICCOPALE(trademark) 100 and PICCOTAC(trademark) 95, or with the aromatic hydrocarbon resin marketed by Hercules under the trade name ENDEX(trademark) 155.
For the purposes of the invention, the aforementioned low-molecular weight organic substances have a molecular weight of up to about 300 and are preferably selected from the group comprising: aromatic and aliphatic hydrocarbons, phenols, alkyl halides, aldehydes, quinones, anhydrides, organic acids, amides, and mixtures thereof.
Among them, the following are preferred: phenanthrene, catechol, resorcinol, iodoform, carbon tetrabromide, m-oxybenzaldehyde, p-nitrobenzaldehyde, p-benzoquinone, succinic anhydride, azelaic acid, caproamide, valeramide, benzamide, and mixtures thereof.